It is sometimes desirable to cause heat affected changes in a selected structure in tissue, such as a vein or hair follicle without causing heat affected changes in tissue adjacent to the selected structure. Selective photothermalysis is a method of irradiating with a laser or pulsed light source that is preferentially absorbed by a pre-selected target. The amount of energy or fluence delivered to the target is chosen such that the temperature rise in the targeted region results in an intended thermal treatment of the target.
Heating of the epidermis may occur during treatment of the target and several methods have been described for cooling the surface of skin during and prior to treatment to minimize the risk of thermal injury to tissue adjacent to the targeted veins. One early method included pre-cooling with ice for several minute prior to treatment. U.S. Pat. No. 5,282,797 issued Feb. 1, 1994 to Chess describes a method of circulating cooling fluid over a transparent plate in contact with the treatment area to cool the epidermis during treatment. U.S. Pat. No. 5,344,418 issued Sep. 6, 1994 to Ghaffari describes a method whereby a coolant is used for a predetermined time interval in coordination with the delivery of laser energy to optimize the cooling of the epidermis and minimize cooling of the targeted vessel. U.S. Pat. No. 5,814,040 issued Sep. 29, 1998 to Nelson et al. describes a cooling method whereby a cryogenic spurt is applied for a predetermined short time directly onto the skin in the target region. The time period for cooling is confined only to the epidermis while leaving the temperature of deeper port wine stains substantially unchanged. Many of the cooling methods may limit the amount of significant thermal damage to the epidermis during treatment.
It may be desirable to shrink collagen in order to reduce the appearance of undesirable conditions of the skin such as acne scars and wrinkles. The following U.S. patents to Sand teach controlled thermal shrinkage of collagen fibers in the cornea using light at wavelengths between 1.8 and 2.55 microns: U.S. Pat. No. 4,976,709, Class No. 606/5, issued Dec. 11, 1990; U.S. Pat. No. 5,137,530; U.S. Pat. No. 5,304,169; U.S. Pat. No. 5,374,265; and U.S. Pat. No. 5,484,432.
U.S. Pat. No. 5,810,801, class no. 606/9 issued Sep. 22, 1998 to Anderson et al. teaches a method and apparatus for treating wrinkles in skin by targeting tissue at a level between 100 microns and 1.2 millimeters below the surface, to thermally injure collagen without causing erythema, by using light at wavelengths between 1.3 and 1.8 microns. Because of the high scattering and absorption coefficients, precooling is utilized to prevent excess heat build up in the epidermis when targeting the region of 100 microns to 1.2 mm below the surface. Specific laser and cooling parameters are selected so as to avoid erythema and achieve improvement in wrinkles as the long term result of a treatment.